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Finding first unique number.

, 24 Jul 2014 CPOL
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Looking at ways to get the first unique number in a sequence.


Recently I took a test to find the first unique number in an array. I did manage to produce the right answer but it was computationally too expensive. This post is the result of looking into alternatives

Original Solution

var y = numberArray.GroupBy(z => z).Where(z => z.Count() == 1)
                                   .Select(z => z.Key).ToList();
 if (y.Count > 0) actual = y[0];


I asked for help on Stack Overflow as I want to see how other devs approached the solution and the following are alternatives where suggested.

  1. Change from y.Count toy.Any
  2. numberArray.GroupBy(g => g).Where(w => w.Count() == 1).Select(s => s.Key).FirstOrDefault();
  3. numberArray.ToLookup(x => x).First(x => x.Count() == 1).Key;
  4. numberArray.GroupBy(x => x).First(x => x.Count() == 1).Key;

In order to make head or tail of the speed difference I decided to run some tests, the template for each test is.

var sw = new System.Diagnostics.StopWatch2() { ShowStatsForEachLoop = false };
int actual = 0;
for (int i = 0; i < Iterations; i++)
     Find comparison here

Test Results

Switching to use .Any() was surprising how big a difference it made. The other surprise was the difference between .GroupBy() and .ToLookup()

If you want speed then you should look at using tradtional loops instead of LINQ.

All solutions where run 50 times with an array of 9 numbers the last number being the unique number. The results below record the average number of CPU ticks for each test.

   Min   Max     Avg  Result
    2  14502     584  -3     Original
    2    919      40  -3     if (y.Any()) actual = y[0];
    2   1423      60  -3     a.GroupBy(g => g).Where(w => w.Count() == 1)
                                            .Select(s => s.Key)
    2   1553      65  -3     a.ToLookup(i => i).First(i => i.Count() == 1).Key;
    2    317      15  -3     a.GroupBy(i => i).First(i => i.Count() == 1).Key;
    1     4        2  -4     Traditonal loops

Source Code

The source code can be found on gitHub @


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Bruce Bennett
Software Developer (Senior)
United Kingdom United Kingdom
No Biography provided

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Comments and Discussions

QuestionJitter time? Pin
Matt T Heffron28-Jul-14 12:48
professionalMatt T Heffron28-Jul-14 12:48 
AnswerRe: Jitter time? Pin
Bruce Bennett28-Jul-14 23:09
memberBruce Bennett28-Jul-14 23:09 
QuestionTraditional? Pin
irneb10-Jul-14 5:42
memberirneb10-Jul-14 5:42 
AnswerRe: Traditional? Pin
Bruce Bennett10-Jul-14 6:22
memberBruce Bennett10-Jul-14 6:22 
GeneralRe: Traditional? Pin
irneb10-Jul-14 18:00
memberirneb10-Jul-14 18:00 
GeneralMy vote of 4 Pin
CatchExAs9-Jul-14 3:51
professionalCatchExAs9-Jul-14 3:51 

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